Facebook urged internet users today not to meet strangers they have only had contact with online after a teenage girl was kidnapped, raped and murdered by a convicted sex offender she met on a social networking site.

The warning came after convicted rapist Peter Chapman, 33 - who was also suspected of other attacks - met 17-year-old Ashleigh Hall online.

Yesterday, Chapman, who admitted kidnap, rape and murder, was jailed for life and told he would serve at least 35 years.

Ashleigh's mother, Andrea Hall, also warned of the dangers of social networking sites and questioned why Chapman was free to attack her daughter.

In a statement, Facebook said it was "deeply saddened" by Ashleigh's death.

The statement urged people not to meet anyone they have only had contact with online unless they are certain they know who they are, "as there are unscrupulous people in the world with malevolent agendas".

Speaking after the sentencing at Teesside Crown Court, Mrs Hall said she had been powerless to stop Ashleigh going out that fateful night.

"Them sort of people should be tagged and they should be kept an eye on all the time," she said.

"I blame them for letting them out. He shouldn't have been let out."

Mrs Hall, who was comforted by her family and friends, said she was powerless to stop Ashleigh going out.

"What could I have done?" she asked.

"She was 17. You don't stop your kids from going out. You tell them to be careful.

"He was a nice-looking boy. I couldn't have stopped it and I wouldn't have stopped it.

"The message is for people just to be careful. Please just be careful. Make sure you please do tell somebody if you are going to meet a person.

"That is the message, don't go on your own."

Her plea was echoed by Chief Superintendent Andy Reddick, who led the murder inquiry.

"The internet is a great thing but young people who go online can easily fall prey to the scheming, devious tricks of predatory paedophiles and perverts who, sadly, are all too willing to prey on the naive and vulnerable," he said.

"It is a jungle that is full of pitfalls and traps and that is why companies who set up and promote such sites should do everything in their power to protect those that use them.

"Parents too should play their part and ensure they know exactly what their children are up to when they are surfing the net.

"Ashleigh made a mistake and paid for it with her life.

"Her death and this murder trial should be a wake-up call to parents and internet companies around the world to ensure as far as they can that nothing happens to another innocent victim."

The Facebook statement read: "While it is not clear how these two met, what is clear is that Peter Chapman was a twisted, determined individual with an evil agenda who used every online and offline opportunity to meet people.

"This case serves as a painful reminder that all internet users must use extreme caution when contacted over the internet by people they do not know.

"We echo the advice of the police, who urge people not to meet anyone they have been contacted by online unless they know for certain who they are, as there are unscrupulous people in the world with malevolent agendas.

"Nothing is more important to Facebook than the safety of the people who use our site. On Facebook there are a variety of measures people can use to protect themselves from unwanted contact and we strongly encourage their use."

Chapman was jailed in 1996 for raping two prostitutes and was also suspected, though never charged, of carrying out other rapes and sex attacks against young women.

And last night Merseyside Police confirmed they waited nine months to put out a national alert after Chapman - who was still on parole - could not be contacted over a traffic offence.

Officers had visited his house in Liverpool in January last year but it took until September for the full alert to be issued.

A spokesman said he did not know why the force had failed to go national earlier.

During the trial, the court heard how Chapman created an alter- ego of 19-year-old Peter Cartwright on the Facebook and Tagged.com websites to groom the Darlington College childcare student.

Once he had made contact - less than a week before her death - he began talking to her using the MSN online messaging service and sending text messages to her mobile phone.

Prosecutor Graham Reeds QC told the court: "It is apparent from the very start that he was obsessed with sex and was trying to make arrangements to meet her."

Mr Reeds said Chapman needed a ruse to ensure Ashleigh got into his car on the night of October 25 last year.

"Having invented 19-year-old Peter to make contact with girls he now decided to invent Peter's dad in order to persuade Ashleigh that it was safe to get into his car," he said.

"The plan he devised was calculated and wicked, and it worked."

Mr Reeds said Chapman had spent the early part of that Sunday visiting the area where he was to eventually take her.

"Having organised the 'Pete's dad' cover story, having researched the location of the intended crime scene and having just travelled to the area to check it, the defendant had now sprung the trap," Mr Reeds said.

"All he had to do now was wait for her to fall into it."

After kidnapping Ashleigh, Chapman - who was living in the back of his car - drove her to an area off the A177 at Thorpe Larches, near Sedgefield, County Durham.

It was there that he forced her to give him oral sex, before he bound and gagged her and raped her.

"After removing her lower clothing she was gagged with duct tape wound around her face, bound by the forearms with tape and forced her into sexual intercourse," Mr Reeds said.

"At some stage the bindings around her arms were removed - most likely to allow her to pull up and fasten her lower clothing.

"But then after the rape her arms were bound up again and further tape was put over her face, suffocating her to death."

Chapman was arrested the following day for motoring offences. He then led police to where he had dumped Ashleigh's body, a nearby farmer's field.

Chapman later confessed to police that he had consensual sex with Ashleigh and she had accidentally suffocated.

Judge Peter Fox, the Recorder of Middlesbrough, jailed Chapman for life and said he would serve at least 35 years' imprisonment before being considered for parole.

"For it appears to me that you are, you were at the time, and have been for some considerable time, a very great danger to young women and, for what it is worth, I cannot foresee your release," he said.

"In my judgment, your killing was of such seriousness on its own, and in conjunction with other associated offences, that it falls clearly into the category of being particularly high.

"This was an evil scheme very carefully brought, and with considerable detail, to trap your victim."

Judge Fox also imposed concurrent 10-year sentences for rape and kidnap but imposed no separate penalty for the offence of failing to notify a change of address.